Exercise 'ping pong'
~~~~~~~~~~~~~~~~~~~~

- Write a program in which two processes repeatedly pass a message back and
  forth.

- Insert timing calls to measure the time taken for one message
  (send+receive).

- Investigate how the time taken varies with the size of the message.

- Use message sizes 1, 2, 4, 8, 16, 32, ..., ~1 million
  or 1, 10, 100, 1000, ..., 1 million

- Loop over message sizes

- In the loop time this block of code with MPI_Wtime()

	MPI_Send(...);
	MPI_Recv(...);

- Print out a table: message_size transfer_time

- Extra task: produce a graphical plot with double logarithmic axes for
  transfer_time(message_size)

- Timers:

	C       : double MPI_Wtime(void);
 	Fortran : real(8), external :: mpi_wtime()

  see also: MPI_Wtick()